1. Field of the Invention
The present invention relates in general to a method of reducing the resistivity of a semiconductor film. More specifically, it relates to a method of activating a p-type compound film to reduce the resistivity thereof by using infrared lasers with longer duration time and high density.
2. Description of the Related Art
The III-V nitrides of wide-band gap such as GaN, InGaN, AlGaN, AlGaInN, etc. and the II-VI group compounds such as Pxe2x80x94ZnSe, ZnMgSe, etc. exhibit excellent semiconductor light emission, and therefore they are widely applied to form semiconductor films for fabricating optoelectronic devices such as Light Emitting Diodes (LEDs) and Laser Diodes (LDs). However, problems and limits have been encountered using such compounds for forming semiconductor films, therefore impeding the progress of optoelectronic industry.
Take gallium nitride (GaN) as an example. Due to a lack of appropriate substrates having lattices matched with gallium nitrides, so far gallium nitride films are formed only on sapphire substrates. For this reason, a gallium nitride film has to be formed using the technique of forming a buffer layer on the sapphire substrate in conjunction with the process of Metal-Organic Chemical Vapor Deposition (hereinafter referred to as MOCVD). When the gallium nitride film is doped with P-type impurity such as Be, Mg, Ca, Zn, Cd, etc. during the film""s growth, the P-type impurity easily reacts with the hydrogen in the reaction gas (metal-organic compound gas). Therefore, the gallium nitride film formed on the sapphire substrate inevitably has high resistivity. The P-type gallium nitride film fabricated according to the conventional art generally has a resistivity higher than 105 xcexa9cm and a hole concentration lower than 1012 cmxe2x88x923. Consequently, the application of the gallium nitride film is rather restricted.
I. Akasaki and H. Amano (NAGOYA university, Japan) disclosed a method which activates the P-type gallium nitride film doped with Mg using Low Energy Electron Beam Irradiation (LEEBI) so as to reduce the resistivity thereof. However, the method can not be effectively applied because the use of LEEBI cannot obtain good throughput due to low rate of activation. Further, this method can only reduce the resistivity of the surface portion of the gallium nitride film.
S. Nakamura (Nichia Chemical Industries, Ltd. Japan) disclosed a method using MOCVD method in conjunction with low-temperature gallium nitride buffer layer to form a P-type gallium nitride film on a substrate. Then, the P-type gallium nitride film is annealed at high temperature according to the conventional art, thereby reducing the resistivity of the P-type gallium nitride film. The annealing process is carried out at a temperature of between 400xcx9c1200xc2x0 C. in the ambience of nitrogen gas. Also, a specific processing temperature between 400xcx9c1200 xc2x0 C. is kept for longer than one minute. Because Nakamura""s method (process) must be carried out in the ambience of nitrogen gas or inert gas instead of in the ambience of atmosphere, the cost and difficulty of the above process rises sharply.
Therefore, an object of the present invention is to provide a novel method to overcome the above problems by using lasers to activate P-type compound semiconductor films for reducing the resistivity thereof.
The first feature of the present invention is that the activating process is performed in the ambience of atmosphere rather than in the ambience of nitrogen gas or inert gas.
The second feature of the present invention is that the time of the activating process can be selected by appropriately adjusting the power density of a laser light source. The power density can be tuned through the control of the emission power of the laser light source, and the focusing distance between the P-type compound semiconductor films and the laser light source.
The present invention achieves the above-indicated object by providing a method of activating P-type compound semiconductor film by using lasers for reducing the resistivity thereof. The method comprises the following steps.
(a) Grow a first P-type compound semiconductor film, wherein the first P-type compound semiconductor film is made from a III-V nitride or a II-VI group compound doped with P-type impurity.
The first P-type compound semiconductor film can be grown using metal-organic chemical vapor deposition (MOCVD) method, molecular beam epitaxy (MBE) method, or hydride vapor phase epitaxy (HVPE) method.
The III-V nitride can be selected from GaN, InGaN, AlGaN, or AlGaInN, and the II-VI group compound can be selected from Pxe2x80x94ZnSe, ZnMgSe, etc.
(b) Expose the first P-type compound semiconductor film to a laser light source (for example, an infrared laser). The laser light source provides energy for breaking the bonds between the P-type impurity in the first P-type compound semiconductor film and the hydrogen so as to release the carriers of the P-type impurity. By this way, the first P-type compound semiconductor film is transformed into a second P-type compound semiconductor film.
It is noted that the resistivity of the second P-type compound semiconductor film is lower than that of the first P-type compound semiconductor film.